Regulated D.C. power supplies provide predictable and reliable voltage sources for driving electronic circuitry. The conventional power supply design typically employs a power device for developing a D.C. voltage output and a regulating feedback loop. The regulation loop serves to maintain the power supply output voltage at a pre-selected set point by sensing the output voltage and increasing or decreasing the output relative to the desired set point.
To dampen the response of the feedback loop, many power supply designs employ a slew rate limiting circuit. The circuit changes the supply output voltage from 0 volts to a preselected set point voltage relatively slowly, rather than instantaneously. As a result, the slew rate circuit tends to reduce the stress on any loads developed by the sudden application of power to a deenergized electronic circuit.
The dampening effect of the slew rate circuit also reduces any transient voltage overshoot associated with the power supply regulation loop. Overshoots often develop from the fast feedback response of the system that produces a high initial error at start-up, causing saturation of the control loop with a corresponding overshoot above the desired set point.
Individual power supplies often have power capabilities corresponding to the physical unit sizes. This is often due to the bulk created by cooling fans, heat sinks and other cooling systems typically required to protect high-power devices from over-heating. An alternative to employing these large, bulky units involves utilizing a plurality of relatively small, low-powered power supply units in parallel relationship. This configuration also provides redundancy in the power supply system for ensuring that a load receives a relatively uninterrupted supply of power.
For power supply systems utilizing a plurality of parallel power supplies, slew rate limiting for each unit becomes relatively complex. It is important to maintain separate slew rate control for the individual power supplies because of inconsistencies in electronic component values that make up each power supply. Moreover, it is desirable to have the capability of selectively bringing one or more power supplies off-line should a problem arise with respect to component overheating or the like.
Therefore, the need exists for a current sharing apparatus and method for use with a power supply system having the capabilities of separately controlling the respective power supply slew rates, and for selectively bringing one or more power supplies off-line without shutting down the entire power supply system. The apparatus and method of the present invention satisfies these needs.